Wireless and Mobile Networks Reading: Sections 2.8 and 4.2.5

Transcription

1 Wireless and Mobile Networks Reading: Sections 2.8 and Acknowledgments: Lecture slides are from Computer networks course thought by Jennifer Rexford at Princeton University. When slides are obtained from other sources, a reference will be noted on the bottom of that slide and full reference details on the last slide. 1

2 Goals of Todayʼs Lecture Wireless links: unique channel characteristics High, time-varying bit-error rate Broadcast where some nodes can t hear each other Mobile hosts: addressing and routing challenges Keeping track of the host s changing attachment point Maintaining a data transfer as the host moves Some specific examples Wireless: wireless LAN (aka WiFi ) Mobility: Boeing Connexion and Mobile IP Many slides adapted from Jim Kuroseʼs lectures at UMass-Amherst 2

3 Widespread Deployment Worldwide cellular subscribers 1993: 34 million 2005: more than 2 billion Now more than landline subscribers Wireless local area networks Wireless adapters built in to most laptops, and even PDAs More than 220,000 known WiFi locations in 134 countries Probably many, many more (e.g., home networks, corporate networks, ) 3

4 Wireless Links and Wireless Networks 4

5 Wireless Links: High Bit Error Rate Decreasing signal strength Disperses as it travels greater distance Attenuates as it passes through matter 5

6 Wireless Links: High Bit Error Rate Interference from other sources Radio sources in same frequency band E.g., 2.4 GHz wireless phone interferes with b wireless LAN Electromagnetic noise (e.g., microwave oven) 6

7 Wireless Links: High Bit Error Rate Multi-path propagation Electromagnetic waves reflect off objects Taking many paths of different lengths Causing blurring of signal at the receiver receiver transmitter 7

8 Dealing With Bit Errors Wireless vs. wired links Wired: most loss is due to congestion Wireless: higher, time-varying bit-error rate Dealing with high bit-error rates Sender could increase transmission power Requires more energy (bad for battery-powered hosts) Creates more interference with other senders Stronger error detection and recovery More powerful error detection codes Link-layer retransmission of corrupted frames 8

9 Wireless Links: Broadcast Limitations Wired broadcast links E.g., Ethernet bridging, in wired LANs All nodes receive transmissions from all other nodes Wireless broadcast: hidden terminal problem C A and B hear each other B and C hear each other But, A and C do not A B So, A and C are unaware of their interference at B. 9

10 Wireless Links: Broadcast Limitations Wired broadcast links E.g., Ethernet bridging, in wired LANs All nodes receive transmissions from all other nodes Wireless broadcast: fading over distance A B C A s signal strength space C s signal strength A and B hear each other B and C hear each other But, A and C do not So, A and C are unaware of their interference at B. 10

11 Example Wireless Link Technologies Data networks Indoor (10-90 meters) a and g: 54 Mbps b: 5-11 Mbps Outdoor (300 meters to 20 kmeters) n: 600 Mbps WiMax: 144/35 (D/U) Mbps Cellular networks, outdoors 3G enhanced: 4 Mbps 3G: 384 Kbps 2G: 56 Kbps 11

12 Wireless Network: Wireless Link network infrastructure Wireless link Typically used to connect mobile(s) to base station Also used as backbone link Multiple access protocol coordinates link access 12

13 Wireless Network: Wireless Hosts network infrastructure Wireless host Laptop, PDA, IP phone Run applications May be stationary (nonmobile) or mobile 13

14 Wireless Network: Base Station network infrastructure Base station Typically connected to wired network Relay responsible for sending packets between wired network and wireless host(s) in its area E.g., cell towers, access points 14

15 Wireless Network: Infrastructure network infrastructure Network infrastructure Larger network with which a wireless host wants to communicate Typically a wired network Provides traditional network services May not always exist 15

16 Scenario #1: Infrastructure Mode network infrastructure Infrastructure mode Base station connects mobiles into wired network Network provides services (addressing, routing, DNS) Handoff: mobile changes base station providing connection to wired network 16

17 Scenario #2: Ad Hoc Networks Ad hoc mode No base stations Nodes can only transmit to other nodes within link coverage Nodes self-organize and route among themselves 17

18 Infrastructure vs. Ad Hoc Infrastructure mode Wireless hosts are associated with a base station Traditional services provided by the connected network E.g., address assignment, routing, and DNS resolution Ad hoc networks Wireless hosts have no infrastructure to connect to Hosts themselves must provide network services Similar in spirit to the difference between Client-server communication Peer-to-peer communication 18

19 Different Types of Wireless Networks Infrastructure-based Single hop Base station connected to larger wired network (e.g., WiFi wireless LAN, and cellular telephony networks) Multi-hop Base station exists, but some nodes must relay through other nodes (e.g., wireless sensor networks, and wireless mesh networks) Infrastructure-less No wired network; one node coordinates the transmissions of the others (e.g., Bluetooth, and ad hoc ) No base station exists, and some nodes must relay through others (e.g., mobile ad hoc networks, like vehicular ad hoc networks) 19

20 WiFi: Wireless LANs 20

21 LAN Architecture AP Internet hub, switch or router Access Point (AP) Base station that communicates with the wireless hosts Basic Service Set (BSS) Coverage of one AP AP acts as the master Identified by a network name known as an SSID BSS 1 AP BSS 2 SSID: Service Set Identifier 21

22 Channels and Association Multiple channels at different frequencies Network administrator chooses frequency for AP Interference if channel is same as neighboring AP Access points send periodic beacon frames Containing AP s name (SSID) and MAC address Host scans channels, listening for beacon frames Host selects an access point to associate with Beacon frames from APs Associate request from host Association response from AP 22

23 Mobility Within the Same Subnet H1 remains in same IP subnet IP address of the host can remain same Ongoing data transfers can continue uninterrupted H1 recognizes the need to change H1 detects a weakening signal Starts scanning for stronger one router Changes APs with same SSID H1 disassociates from one And associates with other Switch learns new location Self-learning mechanism BBS 1 hub or switch AP 1 AP 2 H1 BBS 232

24 CSMA: Carrier Sense, Multiple Access Multiple access: channel is shared medium Station: wireless host or access point Multiple stations may want to transmit at same time Carrier sense: sense channel before sending Station doesn t send when channel is busy To prevent collisions with ongoing transfers But, detecting ongoing transfers isn t always possible C A B C A B A s signal strength C s signal strength space 24

25 CA: Collision Avoidance, Not Detection Collision detection in wired Ethernet Station listens while transmitting Detects collision with other transmission Aborts transmission and tries sending again Problem #1: cannot detect all collisions Hidden terminal problem Fading Problem #2: listening while sending Strength of received signal is much smaller Expensive to build hardware that detects collisions So, does not do collision detection 25

26 Medium Access Control in Collision avoidance, not detection Once a station starts transmitting, send in its entirety More aggressive collision-avoidance techniques E.g., waiting a little after sensing an idle channel To reduce likelihood two stations transmit at once Link-layer acknowledgment and retransmission CRC to detect errors Receiving station sends an acknowledgment Sending station retransmits if no ACK is received Giving up after a few failed transmissions 26

27 Host Mobility 27

28 Varying Degrees of User Mobility Moves only within same access network Single access point: mobility is irrelevant Multiple access points: only link-link layer changes Either way, users is not mobile at the network layer Shuts down between changes access networks Host gets new IP address at the new access network No need to support any ongoing transfers Applications have become good at supporting this Maintains connections while changing networks Surfing the net while driving in a car or flying a plane Need to ensure traffic continues to reach the host 28

29 Maintaining Ongoing Transfers Seamless transmission to a mobile host A B 29

30 E.g., Keep Track of Friends on the Move Sending a letter to a friend who moves often How do you know where to reach him? Option #1: have him update you Friend contacts you on each move So you can mail him directly E.g., Boeing Connexion service Option #2: ask his parents when needed Parents serve as permanent address So they can forward your letter to him E.g., Mobile IP 30

31 Option #1: Let Routing Protocol Handle It Mobile node has a single, persistent address Address injected into routing protocol (e.g., OSPF) A /24 B /32 Mobile host with IP address

32 Example: Boeing Connexion Service Boeing Connexion service Mobile Internet access provider WiFi hot spot at 35,000 feet moving 600 mph Went out of business in December 2006 Communication technology Antenna on the plane to leased satellite transponders Ground stations serve as Internet gateways Using BGP for mobility IP address block per airplane Ground station advertises into BGP 32

33 Example: Boeing Connexion Service /24 Internet 33

34 Summary: Letting Routing Handle It Advantages No changes to the end host Traffic follows an efficient path to new location Disadvantages Does not scale to large number of mobile hosts Large number of routing-protocol messages Larger routing tables to store smaller address blocks Alternative Mobile IP 34

35 Option #2: Home Network and Home Agent Home network: permanent home of mobile (e.g., /24) Home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote Permanent address: address in home network, can always be used to reach mobile e.g., wide area network correspondent Correspondent: wants to communicate with mobile 35

36 Visited Network and Care-of Address Permanent address: remains constant (e.g., ) Visited network: network in which mobile currently resides (e.g., /24) Care-of-address: address in visited network. (e.g., 79, ) wide area network Correspondent: wants to communicate with mobile Foreign agent: entity in visited network that performs mobility functions on behalf of mobile. 36

37 Mobility: Registration home network visited network 2 wide area network foreign agent contacts home agent home: this mobile is resident in my network 1 mobile contacts foreign agent on entering visited network Foreign agent knows about mobile Home agent knows location of mobile 37

38 Mobility via Indirect Routing home network correspondent addresses packets using home address of mobile home agent intercepts packets, forwards to foreign agent 1 wide area network 2 foreign agent receives packets, forwards to mobile 4 3 visited network mobile replies directly to correspondent 38

39 Indirect Routing: Efficiency Issues Mobile uses two addresses Permanent address: used by correspondent (making mobile s location is transparent to correspondent) Care-of-address: used by the home agent to forward datagrams to the mobile Mobile may perform the foreign agent functions Triangle routing is inefficient E.g., correspondent and mobile in the same network 39

40 Mobility via Direct Routing correspondent forwards to foreign agent foreign agent receives packets, forwards to mobile visited network home network 4 wide area 2 network 3 correspondent requests, receives foreign address of mobile 1 4 mobile replies directly to correspondent No longer transparent to the correspondent 40

41 Mobility Today Limited support for mobility E.g., among base stations on a campus Applications increasingly robust under mobility Robust to changes in IP address, and disconnections E.g., client contacting the server and allowing reading/writing while disconnected Increasing demand for seamless IP mobility E.g., continue a VoIP call while on the train Increasing integration of WiFi and cellular E.g., dual-mode cell phones that can use both networks Called Unlicensed Mobile Access (UMA) 41

42 Impact on Higher-Layer Protocols Wireless and mobility change path properties Wireless: higher packet loss, not from congestion Mobility: transient disruptions, and changes in RTT Logically, impact should be minimal Best-effort service model remains unchanged TCP and UDP can (and do) run over wireless, mobile But, performance definitely is affected TCP treats packet loss as a sign of congestion TCP tries to estimate the RTT to drive retransmissions TCP does not perform well under out-of-order packets Internet not designed with these issues in mind 42

43 Conclusions Wireless Already a major way people connect to the Internet Gradually becoming more than just an access network Mobility Today s users tolerate disruptions as they move and applications try to hide the effects Tomorrow s users expect seamless mobility Challenges the design of network protocols Wireless breaks the abstraction of a link, and the assumption that packet loss implies congestion Mobility breaks association of address and location Higher-layer protocols don t perform as well 43